[Zwee42]

Summary

Add the repository's first typed ESLint setup and enforce code quality in CI. React hook mistakes in both .ts and .tsx files now fail the lint check, including missing effect dependencies. JSX accessibility rules remain scoped to .tsx.

CI
  npm ci
  npm run typecheck        # TypeScript 7 application compiler
  npm run typecheck:tests  # TypeScript 6 tests + Vite configuration
  npm run lint
  npm test
  npm run build

• Add the flat ESLint configuration, TypeScript 6 alias for typed linting, and documented lint/typecheck commands. The application compiler remains TypeScript 7.0.2.
• Fix the original lint findings and the findings introduced by the newer tour code; incorporate current main while preserving the label hover and click behavior.
• Complete the structure tree's roving focus: one Tab entry point, Up/Down, parent/child Left/Right, Home/End, keyboard selection, and a visible focus ring. Search selection still expands and reveals the selected item.
• Add seven regression tests covering hook enforcement and tree entry points. Lint fixture tests opt out of single-run parser inference because they repeatedly lint in-memory snippets; the production lint configuration remains type-aware.
• Keep the JSX accessibility plugin's existing ESLint 10 dependency override. Version 6.10.2 declares a peer range ending at ESLint 9; the override is validated by the lint check rather than implying declared upstream support.

Evidence

• Before: .ts hook mistakes were unchecked; a missing .tsx effect dependency produced zero errors and one warning. The initial expanded tree had 20 Tab stops.
  After: Missing dependencies and conditional hooks fail regression tests in both extensions. Browser verification confirms one Tab stop, parent/child navigation, Home/End, Tab exit, selection reveal, and the visible focus ring, without console errors.
• Local validation: npm run lint, both typechecks, all 218 tests (also with CI=true), and npm run build pass.
• GitHub CI: all checks passed for commit cffe9b6, including both typechecks, lint, 218 tests, and build.

Merge Danger

Door: two-way; the change can be reverted.

Blast Radius: tooling and tree keyboard interaction. CI is stricter, and keyboard users traverse the tree using arrows instead of tabbing through every row.

Closes #42.

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